readahead() initiates readahead on a file so that subsequent reads
from that file will be satisfied from the cache, and not block on
disk I/O (assuming the readahead was initiated early enough and that
other activity on the system did not in the meantime flush pages from
the cache).
The fd argument is a file descriptor identifying the file which is to
be read. The offset argument specifies the starting point from which
data is to be read and count specifies the number of bytes to be
read. I/O is performed in whole pages, so that offset is effectively
rounded down to a page boundary and bytes are read up to the next
page boundary greater than or equal to (offset+count). readahead()
does not read beyond the end of the file. The current file offset of
the open file referred to by fd is left unchanged.

readahead() attempts to schedule the reads in the background and
return immediately. However, it may block while it reads the
filesystem metadata needed to locate the requested blocks. This
occurs frequently with ext[234] on large files using indirect blocks
instead of extents, giving the appearance that the call blocks until
the requested data has been read.

This page is part of release 3.82 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2014-03-15 READAHEAD(2)